Compositions and systems for combatting lost circulation and methods of using the same

ABSTRACT

A method of servicing a wellbore in a subterranean formation comprising preparing a composition comprising a base fluid, a thixotropic viscosifier, a gellable composition and a bridging material, applying a shear force to the composition such that the composition viscosity decreases, introducing the composition into a lost circulation zone in the subterranean formation, wherein the lost circulation zone comprises cavities greater than about 200 microns in diameter, decreasing the shear force applied to the composition, and allowing the composition to set in the lost circulation zone. A method of servicing a wellbore in a subterranean formation comprising placing a first stream comprising a dilute solution of a metal acrylate into a lost circulation zone in the subterranean formation, placing a second stream comprising an activator into the lost circulation zone, and forming a lost circulation material upon contacting of the metal acrylate and the activator, wherein the lost circulation material forms in from about 0 to about 60 minutes.

BACKGROUND OF THE INVENTION

This disclosure relates to compositions and systems for servicing awellbore in a subterranean formation. More specifically, this disclosurerelates to introducing compositions and systems into a wellborepenetrating a subterranean formation to reduce the loss of fluid to theformation.

A natural resource such as oil or gas residing in a subterraneanformation may be recovered by drilling a well into the formation. Thesubterranean formation is usually isolated from other formations using atechnique known as well cementing. In particular, a wellbore istypically drilled down to the subterranean formation while circulating adrilling fluid through the wellbore. After the drilling is terminated, astring of pipe, e.g., casing, is run in the wellbore. Primary cementingis then usually performed whereby a cement slurry is pumped down throughthe string of pipe and into the annulus between the string of pipe andthe walls of the wellbore to allow the cement slurry to set into animpermeable cement column and thereby seal the annulus. Subsequentsecondary cementing operations, i.e., any cementing operation after theprimary cementing operation, may also be performed. One example of asecondary cementing operation is squeeze cementing whereby a cementslurry is forced under pressure to areas of lost integrity in theannulus to seal off those areas.

Subsequently, oil or gas residing in the subterranean formation may berecovered by driving the fluid into the well using, for example, apressure gradient that exists between the formation and the wellbore,the force of gravity, displacement of the fluid using a pump or theforce of another fluid injected into the well or an adjacent well. Theproduction of the fluid in the formation may be increased byhydraulically fracturing the formation. That is, a viscous fracturingfluid may pumped down the casing to the formation at a rate and apressure sufficient to form fractures that extend into the formation,providing additional pathways through which the oil or gas can flow tothe well. Unfortunately, water rather than oil or gas may eventually beproduced by the formation through the fractures therein. To provide forthe production of more oil or gas, a fracturing fluid may again bepumped into the formation to form additional fractures therein. However,the previously used fractures first must be plugged to prevent the lossof the fracturing fluid into the formation via those fractures.

In addition to the fracturing fluid, other fluids used in servicing awellbore may also be lost to the subterranean formation whilecirculating the fluids in the wellbore. In particular, the fluids mayenter the subterranean formation via lost circulation zones (LCZs) forexample, depleted zones, zones of relatively low pressure, zones havingnaturally occurring fractures, weak zones having fracture gradientsexceeded by the hydrostatic pressure of the drilling fluid, and soforth. As a result, the service provided by such fluid is more difficultto achieve. For example, a drilling fluid may be lost to the formation,resulting in the circulation of the fluid in the wellbore being too lowto allow for further drilling of the wellbore. Also, a secondarycement/sealant composition may be lost to the formation as it is beingplaced in the wellbore, thereby rendering the secondary operationineffective in maintaining isolation of the formation. Conventionalsolutions to preventing loss of wellbore fluids to an LCZ involveforming a viscous mass in the LCZ. Frequently, the viscous masses areeasily deformable and may breakdown under fluid pressure, therebyallowing reestablishment of a fluid flow channel within the LCZ.Accordingly, an ongoing need exists for more effective compositions andmethods of blocking the flow of fluid through LCZs in subterraneanformations.

SUMMARY

Disclosed herein is a method of servicing a wellbore in a subterraneanformation comprising preparing a composition comprising a base fluid, athixotropic viscosifier, a gellable composition and a bridging material,applying a shear force to the composition such that the compositionviscosity decreases, introducing the composition into a lost circulationzone in the subterranean formation, wherein the lost circulation zonecomprises cavities greater than about 200 microns in diameter,decreasing the shear force applied to the composition, and allowing thecomposition to set in the lost circulation zone.

Also disclosed herein is a method of servicing a wellbore in asubterranean formation comprising placing a first stream comprising adilute solution of a metal acrylate into a lost circulation zone in thesubterranean formation, placing a second stream comprising an activatorinto the lost circulation zone, and forming a lost circulation materialupon contacting of the metal acrylate and the activator, wherein thelost circulation material forms in from about 0 to about 60 minutes.

Further disclosed herein is a method of servicing a wellbore in asubterranean formation comprising placing a composition comprising adilute solution of a cross-linkable material and an encapsulatedactivator into a lost circulation zone in the subterranean formation,and allowing the composition to set.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and theadvantages thereof, reference is now made to the following briefdescription, taken in connection with the accompanying drawings anddetailed description, wherein like reference numerals represent likeparts.

FIG. 1 is an illustration of a wellbore penetrating a subterraneanformation and a lost circulation zone (LCZ) extending into theformation.

FIG. 2 is a rheological profile of the samples from Example 3.

DETAILED DESCRIPTION

It should be understood at the outset that although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

Disclosed herein are wellbore servicing compositions, wellbore servicingsystems, and methods of employing the same. The disclosed compositionsand methods may be utilized to plug and/or seal a LCZ therebypreventing, ceasing or substantially lessening the loss of fluids tosuch a zone. Hereinafter such compositions and systems employed tocombat loss circulation are termed loss circulation combatingcompositions (LCCCs) and are described in greater detail herein.

FIG. 1 depicts an exemplary operating environment of an embodiment ofthe compositions, systems, and methods disclosed herein. It is notedthat although some of the figures may exemplify horizontal or verticalwellbores, the principles of the foregoing process, methods, and systemsare equally applicable to horizontal and vertical conventional wellboreconfigurations. The horizontal or vertical nature of any figure is notto be construed as limiting the wellbore to any particularconfiguration. As depicted, the operating environment comprises adrilling rig 106 that is positioned on the earth's surface 104 andextends over and around a wellbore 114 that penetrates a subterraneanformation 102 for the purpose of recovering hydrocarbons. The wellbore114 may be drilled into the subterranean formation 102 using anysuitable drilling technique. In an embodiment, the drilling rig 106comprises a derrick 108 with a rig floor 110 through which a work string112 extends downward from the drilling rig 106 into the wellbore 114. Inan embodiment, the work string 112 delivers a wellbore servicingapparatus or some part thereof to a predetermined depth within thewellbore 114 to perform a wellbore servicing operation.

The wellbore 114 may extend substantially vertically away from theearth's surface 104 over a vertical wellbore portion, or may deviate atany angle from the earth's surface 104 over a deviated or horizontalwellbore portion. In alternative operating environments, portions orsubstantially all of the wellbore 114 may be vertical, deviated,horizontal, and/or curved. In some instances, at least a portion of thewellbore 114 may be lined with a casing 120 that is secured intoposition against the formation 102 in a conventional manner using cement122. In alternative operating environments, the wellbore 114 may bepartially cased and cemented thereby resulting in a portion of thewellbore 114 being uncased.

During any one or more of wellbore drilling, completion, or servicingoperations, an LCZ 150 as described herein may be encountered. Where anLCZ 150 is encountered, it may be desirable to employ the compositions,systems, and/or methods disclosed herein to prevent, lessen, minimize,or cease the loss of fluids to the LCZ 150. In an embodiment, thecompositions, systems, and methods disclosed herein comprise theplacement of one or more LCCCs within an LCZ 150. The placement of anLCCC within an LCZ may be an effective means of plugging or sealing offthe LCZ 150 and thereby preventing, ceasing or substantially lesseningthe loss of fluids to such an LCZ 150.

The LCCCs disclosed herein may comprise a base fluid, a gellablematerial, and optionally a thixotropic viscosifier. In some embodiments,the LCCC further comprises a bridging material. Such LCCCs may beemployed to combat lost-circulation. The LCCC disclosed herein may befurther characterized as forming a non-flowable, elastic, and/orplugging gel comprising a three-dimensional network based on newcovalent bond formation among the reactant molecules. The LCCC may bedesigned to prevent the premature loss of fluids deep into the formationaway from LCZ prior to formation of a plugging mass.

In an embodiment, the LCCC has a density in the range of from about 0.5g/cc to about 4.0 g/cc, alternatively from about 0.8 g/cc to about 3g/cc, alternatively from about 1.0 g/cc to about 2.5 g/cc. The densitymay be measured by any suitable methodology. For example, the densitymay be measured using a pressurized fluid density balance in accordancewith the American Petroleum Institute (API) method found in APIRecommended Practice 10B, Section 6.

In an embodiment, the LCCC comprises a base fluid, for example, anaqueous base fluid, alternatively, a substantially aqueous base fluid,as will be described herein in greater detail. In an embodiment, asubstantially aqueous base fluid comprises less than about 50% of anonaqueous component, alternatively less than about 45%, 40%, 35%, 30%,25%, 20%, 15%, 10%, 5%, 4%, 3%, 2% or 1% of a nonaqueous component. Inan embodiment, the base fluid may further comprise an inorganicmonovalent salt, multivalent salt, or both. Nonlimiting examples ofsalts suitable for use in such a base fluid include water solublechloride, bromide and carbonate, hydroxide and formate salts of alkaliand alkaline earth metals, zinc bromide, and combinations thereof. Thesalt or salts in the base fluid may be present in an amount ranging fromgreater than about 0% by weight to a saturated salt solution.

In an embodiment, a composition comprising the base fluid of the typedescribed herein may have a yield stress of from about 40 Pascals toabout 40,000 Pascals, alternatively from about 70 Pascals to about15,000 Pascals, alternatively from about 100 Pascals to about 12,000Pascals. Herein yield stress refers to the force that is required on aspecific area to make a material flow at a specific shear rate. It is astress above which the shape of a material changes without a particularvolume change. A greater shear stress indicates that a larger force isrequired to make a material flow. The yield stress is the minimum shearstress required to make a material to plastically deform. High yieldstress indicates that larger forces must be applied to the same samplearea to deform the sample. The yield stress may be measured by aBrookfield YR-1 Yield Stress Rheometer.

In an embodiment, the amount of base fluid present in the LCCC may be ina range of from about 50 to about 95 percent by weight (wt. %) of theLCCC, alternatively, from about 70 wt. % to about 90 wt. %,alternatively, from about 70 wt. % to about 85 wt. %.

In an embodiment, the LCCC comprises an optional thixotropic viscosifiercomposition. The term “thixotropic viscosifier” herein refers to amaterial which, when contacted with the base fluid imparts a yieldstress to the resulting mixture. Additionally, a thixotropic materialrefers to a material that displays or imparts on the fluid to which itis added (for example, the base fluid) a decrease in viscosity over timeat a constant shear rate. For example, the thixotropic viscosifier mayimpart a thixotropic or substantially thixotropic behavior to the basefluid. The thixotropic viscosifier may comprise a shear-thinning orsubstantially shear-thinning material or an additive that will impart ashear-thinning (e.g., pseudoplastic) or substantially shear-thinningbehavior to the fluid to which it is added. A shear-thinning materialshall mean a material which displays or imparts on the fluid to which itis added (for example, the base fluid) a decrease in viscosity at anincreasing shear rate. In either such embodiment, the thixotropicviscosifier, when contacted with the base fluid, will result in acomposition which exhibits a decrease in viscosity upon the applicationof a shear force and an increase in viscosity as the shear force ceasesto be applied (e.g., zero shear viscosity). Such a shearing force may beapplied to the composition (e.g., base fluid and thixotropicviscosifier) by pumping an LCCC comprising a thixotropic viscosifierthrough wellbore servicing equipment, into a wellbore, and eventuallyinto an LCZ 150. Without seeking to be bound by any particular theory,pumping the LCCC containing such a thixotropic viscosifier through thewellbore servicing equipment applies a sufficient shearing force to thethixotropic viscosifier to decrease the viscosity of the LCCC and allowthe LCCC to flow though the wellbore servicing equipment, into thewellbore, and into the LCZ 150. In an embodiment, the thixotropicviscosifier comprises materials which may also act as a component of thegellable system. Alternatively, the thixotropic viscosifier may comprisea stand-alone additive. In an embodiment where the gellable compositiondoes not have a yield stress and has an adjustable gel time that islonger than the time required to place the LCCC in LCZ, the gellablecomposition may be used in combination with a thixotropic viscosifiercomposition. For example, inclusion of one or more bridging particulateswithin the LCCC may necessitate use of a thixotropic viscosifier.

In an embodiment, thixotropic viscosifier may be present in the LCCC ina range of from about 0.1% to about 20% by weight of the base fluid,alternatively, from about 0.3 to about 15%, alternatively, from about0.5 to about 10%.

In an embodiment, a base fluid containing the thixotropic viscosifierhas a zero shear viscosity of from about 100 cp to about 2,000,000 cp,alternatively from about 500 to about 1,000,000 cp, alternatively fromabout 1200 cp to about 500,000 cp when the concentration of thethixotropic viscosifier in the base fluid is in the above mentionedconcentration ranges. Viscosity is a measure of the resistance of amaterial to flow. A material with a high flow resistance displays a highviscosity. For Newtonian fluids, the shear viscosity, usuallyrepresented by μ, is independent of the shear rate. For non-Newtonianfluids, the non-Newtonian viscosity, η, is dependent on the shear rate.

In an embodiment, a composition comprising a base fluid and thixotropicviscosifier both of the type described herein may have a shear stresshistory response ranging from about 1000 to about 10 million,alternatively from about 5,000 to about 500,000, alternatively fromabout 10,000 to about 100,000. The properties of a material may bedependent on its shear stress history, including the magnitudes andduration of the exposure of the material to shear stresses. The shearstress history may be measured by integrating the volume average shearrate (VASR) versus time history, in which the VASR is measured in unitsof per sec, while the time is in seconds. This method is described in anarticle by Walters et. al. entitled “Kinetic Rheology of HydraulicFracturing Fluids” presented at the 2001 Society of Petroleum EngineersAnnual Technical Conference, presentation SPE 71660, and incorporated byreference in its entirety.

In an embodiment, the chemical composition of the thixotropicviscosifier may vary provided that the composition has an operablecombination of the various fluid properties described previously herein.In various embodiments, the thixotropic viscosifier may have a chemicalcomposition comprising a physically cross-linked polymer system, forexample associative polymer thickeners; a mineral-based solid suspensionsuch as a clay, for example bentonite, in water; an emulsion, forexample an alkali swellable aqueous latex; a naturally produced materialsuch bacterial or plant based gums; or any fluid containing a materialor a combination of materials that provides the beneficial rheologicalproperties described herein. In an embodiment the thixotropicviscosifier comprises a synthetic hectorite clay. In some embodiments,the synthetic hectorite clays further comprise inorganic polyphosphatepeptizers.

The thixotropic viscosifier may provide stable viscosity to an LCCC whenat rest as well as thixotropic properties whereby the viscosity of theLCCC is reduced during pumping but returns (e.g., the viscosityrecovers) when the LCCC is static. The thixotropic viscosifier may be inthe form of free-flowing powders, which are easily dispersed in water.Also, the thixotropic viscosifier may be fine-grained with an averageparticle size of less than one micron. A nonlimiting example of asuitable thixotropic viscosifier is a synthetic hectorite clay,commercially available from Halliburton Energy Services, Inc. asTHERMA-VIS.

In an embodiment, the thixotropic viscosifier comprises a synthetichectorite clay comprising silicon dioxide (SiO₂), magnesium oxide (MgO),lithium oxide (Li₂O), and sodium oxide (Na₂O). In such an embodiment,the SiO₂ may by present in a range from about 50% to about 70% by weightof the thixotropic viscosifier on a dry basis, alternatively, about59.5%, the MgO may by present in a range from about 20% to about 35% byweight of the thixotropic viscosifier on a dry basis, alternatively,about 27.5%, the Li₂O may by present in a range from about 0.1% to about2.0% by weight of the thixotropic viscosifier on a dry basis,alternatively, about 0.8%, and the Na₂O may by present in a range fromabout 1% to about 5% by weight of the thixotropic viscosifier on a drybasis, alternatively, about 2.8%.

In another embodiment, a synthetic hectorite clay may comprise SiO₂,MgO, Li₂O, Na₂O and phosphorus pentoxide (P₂O₅). In such an embodiment,the SiO₂ may by present in a range from about 45% to about 65% by weightof the on a dry basis, alternatively, about 54.5%, the MgO may bypresent in a range from about 21% to about 31% by weight of thehectorite on a dry basis, alternatively, about 26.0%, the Li₂O may bypresent in a range from about 0.1% to about 2.0% by weight of thehectorite on a dry basis, alternatively, about 0.8%, the Na₂O may bypresent in a range from about 1% to about 10% by weight of the hectoriteon a dry basis, alternatively, about 5.6%, and the P₂O₅ may by presentin a range from about 1% to about 10% by weight of the on a dry basis,alternatively, about 4.1%.

In still another embodiment, the SiO₂ may be present in an amount ofabout 54.5% by weight of the synthetic hectorite on a dry basis, the MgOmay be present in an amount of about 26.0% by weight of the synthetichectorite on a dry basis, the Li₂O may by in an amount of about 0.8% byweight of the synthetic hectorite on a dry basis, the Na₂O may bypresent in and amount of about 5.6% by weight of the synthetic hectoriteon a dry basis, and the P₂O₅ may by present in an amount of about 4.1%by weight of the synthetic hectorite on a dry basis.

In an embodiment, the thixotropic viscosifier further comprises alayered silicate clay. A nonlimiting, example of a suitable layeredsilicate clay includes a synthetic, inorganic colloid commerciallyavailable as LAPONITE from Rockwood Additives/Southern Clay Products,Inc, in Gonzales, Tex. Additional disclosure on gellable compositionscomprising a thixotropic viscosifier may be found in U.S. Pat. No.6,823,939 which is incorporated herein by reference in its entirety.

In an embodiment the layered silicate clay may be present in thethixotropic viscosifier in a range from about 20% to about 100% based onthe dry weight of thixotropic viscosifier, alternatively, from about 40%to about 80%, alternatively, from about 50% to about 75%.

In an embodiment, the thixotropic viscosifier may comprise associativepolymer thickeners. Associative polymer thickeners are polymers whichare hydrophobically modified with alkyl groups of 2-22 carbon chainlength, and will impart viscosity to a fluid in which those polymers aredissolved especially at low shear rates or in static fluids. Not seekingto be bound by theory, the viscosity build up may be due to theintermolecular hydrophobic association among the hydrophobic groups ofdifferent individual polymers. Such hydrophobic associations are shearsensitive, which means they break reversibly upon shearing, and reformupon reduction in shear rates. The associative links may serve astransient, physical cross-links, thereby increasing the viscosity of andimparting yield stress to the fluid. In an embodiment, the thixotropicviscosifier may further comprise a salt. Such a salt may increase theassociative interactions, thereby further increasing fluid viscosities.Such associative polymers may be nonionic, anionic, cationic orzwitterionic. A nonlimiting example of suitable associative polymersinclude hydrophobically modified polyethyleneoxide polymers,commercially available as OPTIFLO® from Rockwood Additives/Southern ClayProducts, Inc., in Gonzales, Tex.

In an embodiment, the thixotropic viscosifier may comprise acellulose-based polymer. Such cellulose-based polymers may bepseudoplastic. In an embodiment, such cellulose-based polymers may behydrophobically modified to provide associative interactions amongpolymer chains when dissolved in a fluid. Nonlimiting examples ofsuitable cellulose-based polymers include cellulose ethers,methylcellulose, and hypromellose products commercially available asMETHOCEL from Dow Chemical Company in Midland, Mich. Further nonlimitingexamples of suitable cellulose-based polymers includeethylhydroxyethylcellulose, methylhydroxyethylcellulose,hydroxypropylcellulose, and hydroxypropylcellulose. Using such watersoluble associative polymers, solids free fluids of the presentdisclosure can be designed for application when solids free fluids areneeded, for example in loss circulation zones of depleted formationswith high permeabilities.

In an embodiment, the thixotropic viscosifier comprises apolysaccharide. Suitable such polysaccharides include without limitationthose that when dissolved in an aqueous solution exhibit a yield stress.Not seeking to be bound by theory, the yield stress may be the result ofthe rigid polymer chain structure of such saccharides. Nonlimitingexamples of such polysaccharides include bacterial and plant based gums,for example, xanthan, diutan, gellan, gum tragacanth and pestan.

In an embodiment, the thixotropic viscosifier may comprise a thixotropicsilica. An example of thixotropic silica is amorphous, pyrogenic silica.The thixotropic silica particles may be characterized as having adiameter of less than about 100 nm diameters, alternately, less thanabout 50 nanometers, alternately, less than about 40 nm.

Not seeking to be bound by any particular theory, the presence of asmall amount of salt in the base fluid (about 0.5% to about 5% by weightof the base fluid) may enable the thixotropic viscosifier (e.g., asynthetic hectorite clay or a hydrophobically modified polymer) to forma gel in an aqueous liquid and better impart thixotropic propertiesthereto.

In an embodiment, the LCCC comprises a gellable composition. A gellablecomposition suitable for use in this disclosure may form a highlyrubbery and elastic gel upon placement in or after entering the lostcirculation zone and be characterized by an adjustable gel time rangingfrom nearly instantaneous to a few hours. In an embodiment, the gellablecomposition comprises a fluid with no yield stress prior to gelling. Inan embodiment, one or more components of the gellable composition maycontribute a thixotropic characteristic to LCCC. In an embodiment, thegellable composition may have a gel time shorter than the time requiredto place the LCCC in LCZ. When such composition is used, the componentsof the gellable composition may be pumped separately and mixed in ornear the LCZ. Such compositions may exhibit gel times of from about1/1000 to about ½ of the LCCC placement time, alternately of from about1/100 to about 1/10 of the placement time may or may not have yieldstress, may exhibit low viscosities and may or may not comprise athixotropic viscosifier. Any gellable composition compatible with theother components of the LCCC may be utilized. For example, the gellablecomposition may comprise 1) a water-soluble polymerizable monomer witholefinic unsaturation; 2) a cross-linkable, water soluble polymer; 3) awater-compatible, curable monomer/macromer system containing no olefinicunsaturation and capable of forming a cured thermoset polymer; or 4) aninorganic, water soluble gel forming composition, and one or morereagents or chemicals necessary to convert these materials into gelledcompositions. Such materials may include, a cross-linkingmulti-functional monomer, a polymerization initiator, a cross-linkingagent, a curing agent, a gel time moderating agent, a cure activator, orcombinations thereof depending on the cross-linkable materials selected.

In an embodiment, the amount of gellable composition present in the LCCCmay be in a range from about 0.2 wt. % to about 30 wt. % by weight ofthe LCCC, alternatively, from about 0.5 wt. % to about 20 wt. %,alternatively, from about 1 wt. % to about 15 wt. %.

In an embodiment, the gellable composition comprises a dilute solutionof monomeric acrylates of alkaline earth metals and water, hereinaftertermed a low viscosity gel system (LVGS). The LVGS may comprisecross-linkable water soluble polymerizable monomeric acrylates ofalkaline earth metals, a multifunctional unsaturated cross-linkingmonomer, a redox polymerization initiator, a polymerization ratemoderator, and, optionally, a redox polymerization rate retarder or afree radical initiator. The cross-linkable monomeric acrylates may bepresent in the LVGS in an amount from about 1 wt. % to about 30 wt. %,alternatively, about 5 wt. % to about 20 wt. % alternatively about 10wt. % to about 15 wt. % based on weight of base fluid used in the LCCC;and the redox polymerization initiator may be present in an amount fromabout 3 wt. % to about 30 wt. %, alternatively, about 5 wt. % to about20 wt. %, alternatively about 10 wt. % to about 15 wt. % by weight ofacrylate salt. The redox polymerization rate moderator may be present inan amount from about 3 wt. % to about 25 wt. %, alternatively, about 5wt. % to about 20 wt. %, alternatively about 10 wt. % to about 15 wt. %by weight of the acrylate salt. The polymerization rate retarder may bepresent in an amount from about 0.01 wt. % to about 2 wt. %,alternatively, about 0.05 wt. % to about 1 wt. %, alternatively about0.1 wt. % to about 0.5 wt. % based on the acrylate salt. The freeradical initiator, may be present in an amount from about 0.1 wt. % toabout 5 wt. %, alternatively, about 0.5 wt. % to about 3 wt. %,alternatively about 1 wt. % to about 2 wt. % by weight of the acrylatesalt.

Non-limiting examples of a water soluble, polymerizable monomericacrylates suitable for use in the LVGS include calcium and magnesiumsalts of acrylic acid, salts of methacrylic acid, and combinationsthereof. Non-limiting examples of redox initiators suitable for use inthe LVGS include sodium persulfate (Na₂S₂O₈), ammonium persulfate,sodium percarbonate (Na₂CO₃), sodium perborate (NaBO₃), and combinationsthereof. A non-limiting example of a polymerization rate moderatorsuitable for use in combination with a redox initiator include monomeric(e.g., morpholine), oligomeric (e.g., tetramethylethylenediamine) orpolymeric amines (polyethyelenimine), alkanolamines (e.g.,triethanolamine), ammonium salts (e.g., ammonium carbonate).Non-limiting examples of free radical initiators suitable for use in theLVGS include azo-initiators, peroxoide and hydroperoxides. Cross-linkingmultifunctional monomers suitable for use in combination with the LVGScomposition include without limitation ethylene bisacrylamide, methylenebisacrylamide, trimethylol propane triacrylate, trimethylol propylenediacrylate, ethyleneglycol triacrylate, pentaerythrytol triacrylate,pentaerythrytol diacrylate, triallylcyanurate and mixtures thereof. Theamount of the cross-linking monomer may range from about 0.01 wt. % toabout 5 wt. %, alternately, from about 0.1 wt. % to about 2 wt. % byweight of the polymerizable monomers. In an embodiment, thecross-linkable monomeric acrylates are activated by a mixture comprisinga redox initiator, and a polymerization rate moderator. Upon activation,the cross-linkable composition begins to form a gel. Adjusting the ratioof polymerization rate moderator to redox initiator will vary the geltime achieved upon activation. In an embodiment, the ratio ofpolymerization rate moderator to redox initiator to polymerizationmoderator is in a range from about 1 to 10 to about 10 to 1,alternatively, the ratio is about 1 to 1 by weight. Alternately, theredox initiator may be encapsulated, and used with or without apolymerization rate modifier. Examples of encapsulated redox initiatorsare OPTIFLO II and OPTIFLO III which are commercially from HalliburtonEnergy Services, Inc. A nonlimiting example of polymerization rateretarder is potassium ferricyanide.

While redox types of initiators are suitable for gellation at lowtemperatures, gellation at high temperatures can be achieved by usingfree radical initiators which have free-radical generating decompositiontemperatures in the desired range. Such initiators include water solubleor water dispersible azo initiators and organic peroxides andhydroperoxides. Examples of suitable azo initiators include2,2′-azobis(2-(2-imidazolin-2-yl)propane) dihydrochloride and2,2′-azobis(2-methyl-N-2-hydroxyethyl)propionamide. Suitable watersoluble azo initiators are available commercially from Wako ChemicalsUSA, Inc, Richmond, Va. and from Halliburton Energy Services, Inc. underthe trade name PERM C and PERM D. Nonlimiting examples of suitableorganic peroxides and hydroperoxides include benzoyl peroxide andtert-butyl hydroperoxide.

In an alternative embodiment, the gellable composition comprisescrosslinkable materials comprising self-crosslinking materials and acatalyst. In some embodiments, the density of the LVGS solution may beadjusted by using an aqueous salt solution (e.g., a brine) as a carrierfluid or by the addition of suitable salts in desired amounts. Examplesof brines suitable for use in the LVGS include without limitation zincbromide, calcium chloride, sodium chloride, potassium chloride andcombinations thereof. In an embodiment, the brine comprises zincbromide. The brine may have a density in the range of from about 9 toabout 19 pounds per gallon and may function to modulate the density ofthe composition. Further description of LVGS type suitable for use inthis disclosure can be found in U.S. Pat. Nos. 5,358,051; 6,936,574;6,187,839 and 5,335,726, each of which is incorporated by referenceherein in its entirety.

In an embodiment, the LCCC includes a gel system comprising one or morecrosslinkable polymers and a crosslinking agent, hereinafter termed acrosslinkable material gel system (CMGS-1). The crosslinkable polymericmaterial may be present in the CGMS-1 an amount from about 0.3% to about10%, alternatively, about 0.5 to 5% by weight of the CMGS-1; and thecrosslinking agent may be present in an amount from about 0.01% to about5%, alternatively, about 0.05% by weight of the CMGS-1.

Examples of crosslinkable polymeric materials suitable for use in theCMGS-1 include, but are not limited to a water-soluble copolymer of anon-acidic ethylenically unsaturated polar monomer and an ethylenicallyunsaturated ester monomer; a terpolymer or tetrapolymer of a non-acidicethylenically unsaturated polar monomer, an ethylenically unsaturatedester monomer, and at least one monomer selected from alkali, alkalineearth or ammonium salts of 2-acrylamido-2-methylpropane sulfonic acid,acrylic acid, or both; or combinations thereof. The copolymer maycontain from one to three polar monomers and from one to threeunsaturated esters.

The ethylenically unsaturated ester monomers used in the crosslinkablematerial may be formed from a hydroxyl compound and an ethylenicallyunsaturated carboxylic acid selected from the group consisting ofacrylic, methacrylic, crotonic, and cinnamic acids. The ethylenicallyunsaturated group may be in the alpha-beta or beta-gamma positionrelative to the carboxyl group, but it may be at a further distance. Inan embodiment, the hydroxyl compound is an alcohol generally representedby the formula ROH, wherein R is an alkyl, alkenyl, cycloalkyl, aryl,arylalkyl, aromatic, or heterocyclic group that may be substituted withone or more of a hydroxyl, ether, or thioether group. The substituentcan be on the same carbon atom of the R group as is bonded to thehydroxyl group in the hydroxyl compound. The hydroxyl compound may be aprimary, secondary, iso, or tertiary compound. In an embodiment, atertiary carbon atom is bonded to the hydroxyl group, e.g., t-butyl andtrityl. In an embodiment, the ethylenically unsaturated ester is t-butylacrylate.

The non-acidic ethylenically unsaturated polar monomers used in thecrosslinkable material can be amides (e.g., primary, secondary, and/ortertiary amides) of an unsaturated carboxylic acid. Alternately, theamide group may be part of a cyclic structure. The amide may be derivedfrom ammonia, or a primary or secondary alkylamine, which may beoptionally substituted by at least one hydroxyl group as inalkanolacrylamides such as N-ethylolacrylethamides orN-methylolacrylamides. Examples of such carboxylic derived ethylenicallyunsaturated polar monomers are acrylamide, methacrylamide,N-vinylpyrrolidone, and acrylic ethanol amide.

A crosslinking agent is herein defined as a material that is capable ofcrosslinking polymers to form a gel. Non-limiting examples of acrosslinking agent which may be suitably employed in the CMGS-1 includean organic crosslinking agent such as a polyalkyleneimine, apolyfunctional aliphatic amine such as polyalkylenepolyamine, anaralkylpolyamine, a heteroaralkylpolyamine, or combinations thereof.Examples of suitable polyalkyleneimines are polymerized ethyleneimineand propyleneimine. Examples of suitable polyalkylenepolyamines arepolyethylene- and polypropylene-polyamines. A description of suchpolymers and crosslinking agents can be found in U.S. Pat. Nos.5,836,392, 6,192,986, and 6,196,317, each of which is incorporated byreference herein in its entirety. Other examples of cross-linking agentsinclude transition metal based cross-linking agents. Examples of suchcross-linking agents include chromium (III) compounds, titanic andzirconium complexes of organic ligands.

In an embodiment, the crosslinkable polymer system is a copolymer ofacrylamide and t-butyl acrylate, and the crosslinking agent ispolyethyleneimine. These materials are commercially available as theH2ZERO from Halliburton Energy Services, Inc. The H2ZERO system is acombination of HZ-10 polymer and HZ-20 crosslinker. HZ-10 is apoly(acrylamide-co-acrylate ester) copolymer. HZ-20 polymer is apolyethyleneimine crosslinker. The gelation rate of the H2ZERO system iscontrolled by controlling the ratio of HZ-10 to HZ-20 or by the additionof cross-linking rate modifiers. Suitable crosslinker rate modifiers aredisclosed in U.S. Pat. No. 7,287,587, which is incorporated herein inits entirety. Thus, the concentrations of both HZ-10 polymer and HZ-20crosslinker contribute to the gel system reaction time, its finalmechanical properties and stability. In an embodiment, the crosslinkablepolymer system forms a viscous gel in from about 60 mins to about 300mins, alternatively in from about 120 mins to about 240 mins at atemperature of from about 180° F. to about 320° F., alternatively fromabout 180° F. to about 225° F. and, alternatively from about 250° F. toabout 320° F.

In an embodiment of the LCCC comprises a CMGS-1 which comprises acrosslinkable material present in an amount of from about 1 wt. % toabout 8 wt. % by weight of the LCCC, alternatively from about 1 wt. % toabout 5 wt. %, alternatively from about 2 wt. % to about 4 wt. %; and acrosslinking agent present in an amount of from about 0.1 wt. % to about5 wt. % by weight of the LCCC, alternatively from about 0.3 wt. % toabout 2 wt. %, alternatively from about 0.5 wt. % to about 1 wt. %.

In an embodiment, the gel system comprises water, a cross-linking agent,and a water-soluble polymer or mixture of polymers. Alternatively, thegel system comprises water, an oxidized chitosan-based compound, and awater-soluble compound having carbonyl groups. Alternatively, the gelsystem comprises water, an amine-based polymer, a polysaccharide-basedpolymer, and an oxidizing agent that is capable of at least partiallyoxidizing the polysaccharide-based polymer. Alternatively, the gelsystem comprises a water-soluble polymer having acylated amine unitsthat act as a cross-linker and a crosslinkable water-soluble polymerthat comprises a functional group selected from the group consisting ofcarboxylic acids and carboxylic acid derivatives. Alternatively, the gelsystem comprises water; a water-soluble polymer comprising polymerizedvinyl amine units; and an organic compound capable of crosslinking withthe vinyl amine units of the water-soluble polymer. Alternatively, thegel system comprises an acrylamide polymer and a crosslinking agent.Further descriptions of such gel systems can be found in U.S. Pat. Nos.6,176,345; 6,607,305; 6,843,841; 6,764,981; 6,321,841; 6,192,986;5,836,392; 6,176,315; 7,331,390; 7,128,148; 4,629,747; and 4,683,949each of which is incorporated herein by reference in its entirety.

In an embodiment, the gel system comprises a water-compatible, thermosetpolymer-forming monomer/macromer system containing no olefinicunsaturation, referred to herein as a resin-based gel system. As usedherein, the term “water compatible” means water soluble, waterdilutable, or water dispersible and that the presence of water does notdetrimentally affect the curing reactions. In an embodiment, theresin-based gel system comprises a resin formed from a formaldehydecondensation reaction with an amino resin or a furan resin, a curingagent for causing the LCCC to cure, a coupling agent for bonding theLCCC in a subterranean zone, and, optionally, a diluent. Examples ofamino resin/formaldehyde condensation resins include urea-formaldehyderesins, melamine-formaldehyde resins. Examples of furan/formaldehydyderesins include furfuryl alcohol/formaldehyde condensation resins. In anembodiment, the amino resin/formaldehyde resins and furfurylalcohol/formaldehyde resins may be partially prepolymerized whileretaining water compatibility. Preparation of water soluble furfurylalcohol/formaldehyde resins is described in U.S. Pat. No. 5,486,557, andis incorporated herein in its entirety. Examples of water compatibleresins derived from amino resin/formaldehyde condensation are describedin U.S. Pat. No. 6,881,708. The resin may be present in the gellingcomposition in an amount of from about 5% to about 40%, alternatively,about 10 to about 30%, alternatively about 15 to about 25% by weight ofthe water-compatible, thermoset polymer forming monomer/macromer gelsystem; the curing agent may be present in an amount from about 1% toabout 10%, alternatively, about 2% to about 8%, alternatively about 3%to about 5% by weight of the resin; the coupling agent may be present inan amount from about 0.1% to about 5%, alternatively, about 1% to about4%, alternatively about 2% to about 4% by weight of the resin; thediluent may be present in an amount from about 0% to about 25%,alternatively, about 5% to about 20%, alternatively about 10% to about15% by weight of the resin.

The resin-based gel system may comprise polymerized furfurylalcohol/aldehyde (i.e., furan-formaldehyde polymer) and non-polymerized(i.e., unreacted) furfuryl alcohol/aldehyde. The weight ratio ofpolymerized furfuryl alcohol/aldehyde to non-polymerized or unreactedfurfuryl alcohol/aldehyde in the LCCC may be chosen such that dilutionwith an aqueous fluid does not cause phase separation of the resin andaqueous fluid, as may be determined experimentally for resins withvarying degrees of polymerization.

As mentioned above, the resin-based gelling system may comprise a curingagent, also known as a catalyst. A curing agent is herein defined as amaterial having the ability to cause the resin to cure after a latentperiod to a hard, resilient solid. As used herein, curing refers topolymerizing the non-polymerized resin fraction, as well as furtherpolymerization of previously polymerized resin fraction, thereby forminga crosslinked network of polymer chains. Curing agents suitable forcuring the partially polymerized furfuryl alcohol/aldehyde attemperatures above about 200° F. include, but are not limited to,organic and inorganic acid anhydrides, ammonium salts, sodium bisulfate,hydrolyzable esters such as butyl acetate, furfuryl acetate, organicacids such as maleic acid, fumaric acid, para-toluene sulfonic acid,inorganic acids such as phosphoric or sulfonic acid, and combinationsthereof. In an embodiment, the curing agent comprises an organic acid;alternatively the curing agent comprises sodium bisulfate.

The resin system may also comprise a coupling agent, which is defined asa material having the ability to bond (i.e., adhere) the LCCC(containing the resin composition) to solid surfaces, such as thesurfaces of a metal pipe and of a subterranean well bore and/or thesurfaces within an LCZ or flowpath, when the LCCCs are in a static state(e.g., when the composition is setting). Examples of suitable couplingagents include, but are not limited to, silanes having functional groupsthat give the silanes the ability to bond with solid surfaces. Examplesof such silanes are acrylate functionalized silanes, aminefunctionalized silanes, and vinyl functionalized silanes. Specificexamples of silane coupling agents that can be utilized in the LCCCinclude, but are not limited to,N-2-(aminoethyl)-3-aminopropyltrimethoxysilane,3-glycidoxypropyltrimethoxysilane, andN-beta-(aminoethyl)-gamma-aminopropyl trimethoxysilane,gamma-aminopropyltriethoxysilane, or combinations thereof. In anembodiment, the coupling agent comprisesN-beta-(aminoethyl)-gamma-aminopropyl trimethoxysilane.

A liquid diluent may also be present in the resin system to increase theflexibility and reduce the brittleness of the cured thermoset polymer.Due to the presence of the diluent in the resin system, the degree ofcuring may be relatively reduced. The less expensive diluent thusreduces the overall cost of the LCCC. A diluent suitable for use in thisdisclosure may decrease the viscosity of the LCCC, ensuring that thecompositions can be pumped into a well bore. Further, the diluent mayreduce the brittleness of the LCCC, meaning that it reduces the tendencyof the compositions to crack or flake when bent, flexed, or scratched.The diluent may also act as a heat sink for the exothermic reaction thatoccurs as the non-polymerized resin in the resin system is cured.Examples of diluents for use in this disclosure include, but are notlimited to, alky acetates such as butyl acetate and furfuryl acetate,2-butoxy ethanol, and combinations thereof. In an embodiment the diluentcomprises, butyl acetate, alternatively furfuryl acetate.

The resin system can also comprise a ductility imparting agent. Aductility imparting agent is herein defined as a material having theability to increase the ductility of the cured LCCC (comprising acurable resin system), wherein ductility refers to the ability of amaterial to stretch under the application of tensile load and retain thedeformed shape on the removal of the load. Examples of suitableductility imparting agents include, but are not limited to, phthalatematerials, i.e., organic liquids that cause the curable resin tocrosslink less tightly than normal. Examples of phthalate materialsinclude alkyl phthalates such as diethyl phthalate, butyl benzylphthalate, and di-(2-ethylhexyl) phthalate, or combinations thereof. Inan embodiment, the ductility imparting agent comprises diethylphthalate.

The resin system can be cured at temperatures of from about 80° F. toabove about 200° F., i.e., typical temperatures in a well bore. The curetime of the resin at such temperatures is in the range of from about 6to about 96 hours, allowing it to be placed into a wellbore withoutbeing concerned that it will harden before it reaches its intendedlocation. In addition, the resin system forms a thermoset resin and thuscannot be re-softened despite being exposed to relatively hightemperatures such as those experienced in a well bore. The polymerizedresin system is substantially resistant to degradation by chemicals.

In an alternative embodiment, the gel system comprises a pumpable,corrosion resistant, water compatible hardenable epoxy sealingcomposition. Additional disclosure regarding epoxy based-type systemsmay be found in U.S. Pat. Nos. 6,951,250 and 6,321,841, each of which isincorporated by reference herein in its entirety.

In an embodiment, the gel system comprises a water-soluble salt and anactivator, hereinafter termed a water-soluble salt gel system (WSGS).The water-soluble salt may be present in the WSGS in an amount fromabout 2% to about 25%, alternatively, about 5 to about 20%,alternatively about 8 to about 15% by weight of the WSGS and theactivator may be present in an amount from about 0.5% to about 25%,alternatively, about 1% to about 15%, alternatively about 5% to about10% by weight of the WSGS.

Examples of water-soluble salts suitable for use in a WSGS includewithout limitation an alkali metal metasilicate compound, an alkalimetal silicate compound, an alkali metal aluminate, or combinationsthereof. In an embodiment, the water-soluble salt comprises sodiumsilicate, potassium silicate, sodium aluminate, or combinations thereof,alternatively sodium silicate. Examples of activators suitable for usein this disclosure include without limitation, ethyl acetate, urea,sugar, sodium acid pyrophosphate, chloride, acetate and nitrate salts ofalkali and alkaline earth metals, or combinations thereof.

Not seeking to be bound by any particular theory, reaction of theactivator and the water-soluble salt, for example sodium silicate, mayform an insoluble silica gel or metal silicate matrix. Additionaldisclosure on WSGS may be found in U.S. Pat. Publication No2006/0086501A1 which is incorporated herein by reference in itsentirety.

In an embodiment, the LCCC may advantageously comprise a bridgingmaterial. The bridging material may generally comprise a solid,semi-solid, or particulate material. Again, not seeking to be bound byany particular theory, the bridging material may function to fill,partially fill, bridge, or seal pores and cracks in the LCZ while thegel of the LCCC sets, thereby minimizing fluid loss to the formationuntil the LCCC has gelled completely. Thus, the bridging material mayhold the LCCC in place while gellation occurs. Various bridgingmaterials are set forth herein below.

In an embodiment, the bridging material comprises a filler. Herein, afiller refers to particulate material, also termed finer fillermaterial, designed to bridge off across the packing agent of thebridging material. Such fillers may be smaller in size than the packingagent. In an embodiment, the filler has a specific gravity of less thanabout 0.6 to about 5, alternatively from about 1.5 to about 5,alternatively from about 1.75 to about 4. Without wishing to be limitedby theory, fillers having a specific gravity in the disclosed range mayproduce a bridging material with greater flexibility and ductility.

Examples of suitable fillers include without limitation alkyl quaternaryammonium montmorillonite, bentonite, zeolites, barite, fly ash, calciumsulfate, hollow glass, elastomer, or ceramic beads, and combinationsthereof. In an embodiment the filler is an alkyl quarternary ammoniummontmorillonite. In an embodiment, the filler is a water swellable orhydratable clay. In an alternative embodiment, the filler is a sealingcomposition comprising a hydratable polymer, an organophilic clay and awater swellable clay suspended in a non-aqueous carrier fluid such asmineral oil or kerosene, and contacted with the LCCC in or near LCZ.Such oil-based sealing compositions are disclosed in U.S. Pat. Nos.5,913,364; 6,167,967; 6,258,757, and 6,762,156, each of which isincorporated by reference herein in its entirety. In an embodiment, thefiller material is FLEXPLUG sealant, which is a deformable, viscous,cohesive oil-based composition comprising alkyl quaternary ammoniummontmorillonite commercially available from Halliburton Energy Services,Inc.

In an embodiment, the bridging material comprises a packing agent.Nonlimiting examples of packing agents include without limitationresilient materials such as graphite; fibrous materials such as cedarbark, shredded cane stalks and mineral fiber; flaky materials such asmica flakes and pieces of plastic or cellophane sheeting; and granularmaterials such as ground and sized limestone or marble, vitrified shale,wood, nut hulls, formica, corncobs, gravel, cotton hulls, andcombinations thereof. A resilient graphite material is commerciallyavailable as Duo-Squeeze H from Halliburton Energy Services, Inc. In anembodiment, the packing agent is a resilient graphite such as STEELSEALlost circulation additives which are dual composition graphitederivatives commercially available from Baroid Industrial DrillingProducts, a Halliburton Energy Services, Inc.

In another embodiment, the packing agent is a resin-coated particulate.Examples of suitable resin-coated particulates include withoutlimitation resin-coated ground marble, resin-coated limestone,resin-coated sand, and combinations thereof. In an embodiment, thepacking agent is a resin-coated sand. The sand may be graded sand thatis sized based on a knowledge of the size of the lost circulation zone.The graded sand may have a particle size in the range of from about 10to about 70 mesh, U.S. Sieve Series. The graded sand can be coated witha curable resin, a tackifying agent or mixtures thereof. The hardenableresin compositions useful for coating sand and consolidating it into ahard fluid permeable mass generally comprise a hardenable organic resinand a resin-to-sand coupling agent. Such resin compositions are wellknown to those skilled in the art, as is their use for consolidatingsand into hard fluid permeable masses. A number of such compositions aredescribed in detail in U.S. Pat. Nos. 4,042,032, 4,070,865, 4,829,100,5,058,676 and 5,128,390 each of which is incorporated herein byreference in its entirety. Methods and conditions for the production anduse of such resin coated particulates are disclosed in U.S. Pat. Nos.6,755,245; 6,866,099; 6,776,236; 6,742,590; 6,446,722, and 6,427,775,each of which is incorporated herein by reference in its entirety. Anexample of a resin suitable for coating the particulate includes withoutlimitation SANDWEDGE NT conductivity enhancement system that is a resincoating commercially available from Halliburton Energy Services, Inc.

In some embodiments, the bridging material is a particulate materialsuch as cement. In embodiments, where the bridging material comprises acement, any suitable cement known in the art may be used in the LCCC. Anexample of a suitable cement includes hydraulic cement, which comprisescalcium, aluminum, silicon, oxygen, and/or sulfur and which sets andhardens by reaction with water. Examples of hydraulic cements include,but are not limited to a Portland cement, a pozzolan cement, a gypsumcement, a high alumina content cement, a silica cement, a highalkalinity cement, or combinations thereof. In an embodiment thehydraulic cements are Portland cements of the type described in AmericanPetroleum Institute (API) Specification 10, 5^(th) Edition, Jul. 1,1990, which is incorporated by reference herein in its entirety. Thecement may be, for example, a class A, B, C, G, or H Portland cement.Another example of a suitable cement is microfine cement, for example,MICRODUR RU microfine cement available from Dyckerhoff GmBH ofLengerich, Germany.

In an embodiment, the bridging material is a water-swellable starch.Examples of water-swellable starches suitable for use as a bridgingmaterial include without limitation cross-linked starches. In anembodiment, the bridging material is a granular starch or mixture ofstarches. Alternatively, the bridging material is a pre-gelatinizedstarch. Pre-gelatinized starches may be obtained commercially or theymay be prepared by pre-gelatinization treatment. For pre-gelatinization,the chosen starch granules are heated in water to a point where thestarch granules swell irreversibly. Upon cooling, this swollen structureis retained. The use of pre-gelatinized starches may be advantageous,since these materials are stable at higher temperatures in theformation, e.g., up to 300° F. Chemically modified starches are thosederived from natural starches by chemical reaction of a natural starchwith a suitable organic reactant. Examples of suitable chemicallymodified starches include, but are not limited to, carboxymethyl starch,hydroxyethyl starch, hydroxypropyl starch, acetate starch, sulfamatestarch, phosphate starch, nitrogen modified starch, starch crosslinkedwith aldehydes, epichlorohydrin, borates, and phosphates, and starchesgrafted with acrylonitrile, acrylamide, acrylic acid, methacrylic acid,maleic anhydride, styrene, and combinations thereof. In an embodiment,the starch is present in the composition in an amount effective toprevent leak-off of the gel material. Effective amounts may bedetermined by one of ordinary skill in the art.

In an embodiment, the LCCC may further comprise one or more additives ormodifying agents. Such additives may include but are in no way limitedto fluid absorbing materials, resins, aqueous superabsorbers,viscosifying agents, suspending agents, dispersing agents, salts,accelerants, surfactants, retardants, defoamers, settling preventionagents, weighting materials, dispersants, vitrified shale, formationconditioning agents, or combinations thereof. Other mechanical propertymodifying additives, for example, are carbon fibers, glass fibers, metalfibers, minerals fibers, and the like which can be added to furthermodify the mechanical properties. These additives may be includedsingularly or in combination. Methods for introducing these additivesand their effective amounts, as well as methods of incorporating theseadditives into the LCCC, are known to those of ordinary skill in theart.

Other particulate material may be used in the LCCC alone or incombination with a cement of the type previously described herein. Theparticulate material may be an inert material, and may be sized (e.g., asuitable particle size distribution) based upon the characteristics ofthe void space to be sealed. Examples of suitable particulate materialinclude, but are not limited to, cement, sand, silica flour, gilsonite,graphite; fibrous materials such as cedar bark, shredded cane stalks andmineral fiber; flaky materials such as mica flakes and pieces of plasticor cellophane sheeting, ground battery casings, ground rubber tires; andgranular materials such as ground and sized limestone or marble, wood,nut hulls, formica, corncobs, gravel, ground battery casings, groundrubber tires, cotton hulls, and combinations thereof.

In an embodiments, an LCCC of the type described herein without thebridging agent may be characterized by a viscosity of from about 5centipoise to about 5000 centipoise; alternatively from about 100centipoise to about 2000 centipoise; alternatively from about 300centipoise to about 1.

In an embodiments, an LCCC of the type described herein may becharacterized by a pump time of from about 30 min to about 48 hours;alternatively from about 2 hours to about 24 hours; alternatively fromabout 3 hours to about 18 hours.

In an embodiments, an LCCC of the type described herein may becharacterized by a set time or gel time of from about 0 hours to about96 hours; alternatively from about 0.5 hours to about 24 hours;alternatively from about 3 hours to about 18 hours.

In an embodiment, the LCCC may be employed as a wellbore servicingfluid. As used herein, a “servicing fluid” refers to a fluid used todrill, complete, work over, fracture, repair, or in any way prepare orrestore a wellbore for the recovery of materials residing in asubterranean formation penetrated by the wellbore. Examples of servicingfluids include, but are not limited to, cement slurries, drilling fluidsor muds, spacer fluids, lost circulation materials, fracturing fluids orcompletion fluids, all of which are well known in the art. The servicingfluid is for use in a wellbore that penetrates a subterranean formation.It is to be understood that “subterranean formation” encompasses bothareas below exposed earth and areas below earth covered by water such asocean or fresh water.

In an embodiment, the method of utilizing one or more of the LCCCsdescribed herein may comprise introducing the LCCC into a wellbore;placing at least a portion of the LCCC within a LCZ or other flowpaththrough which the flow of fluids may be desirably reduced or ceased; andcausing or allowing the LCCC to gel.

As noted above, in an embodiment, the LCCCs may be introduced to thewellbore to prevent the loss of aqueous or non-aqueous drilling fluidsinto LCZs such as voids, vugular zones, and natural or induced fractureswhile drilling. In an embodiment, the LCCCs may be introduced to preventthe loss or migration of fluid into lost circulation zones orundesirable flowpaths such as voids, vugular zones, and natural orinduced fractures in the formation. In order to prevent settling ofparticulate materials during pumping a separate settling preventionagent may be included in the LCCC. Nonlimiting examples of commonly usedsettling prevention agents include polymeric agents such asgalactomannans, modified or derivatized galactomannans, cellulosederivatives, and combinations thereof. Cross-linking agents, breakersand other additives may also be included in the LCCC.

In an embodiment, the LCCC is placed into a wellbore as a single streamand activated by downhole conditions to form a barrier thatsubstantially seal a lost circulation zones or other undesirableflowpath. In such an embodiment, the LCCC may be placed downhole throughthe drill bit forming a composition that substantially eliminates thelost circulation. In yet another embodiment, the LCCC is formed downholeby the mixing of a first stream comprising one or more LCCC componentsand a second stream comprising additional LCCC components. For example,the LCCC may be formed downhole by the mixing of a first streamcomprising a cross-linkable polymer and a second stream comprising aninitiator. Methods for introducing compositions into a wellbore to sealsubterranean zones are described in U.S. Pat. Nos. 5,913,364; 6,167,967;and 6,258,757, each of which is incorporated by reference herein in itsentirety.

The LCCCs of this disclosure may provide lost circulation control in asufficiently short time period to prevent the operator from pulling outof the hole and thus reducing nonproductive rig time; various methods ofintroducing the LCCC or components thereof, which will be described ingreater detail herein below, may allow this to be accomplished. In anembodiment, the gellable composition of the LCCC may set or begin to setinstantaneously or substantially instantaneously upon entering the LCZ150. Alternatively, some amount of time may be required before thegellable composition of the LCCC sets or begins to set. The amount oftime between when the LCCC is introduced to a LCZ and sets to reduce orprevent lost circulation may be adjusted by one of ordinary skill in theart with the benefits of this disclosure. In an embodiment, the LCCC maybe designed to set after some amount of time. For example, the gellablecomposition of the LCCC may being to set within about 1 minute,alternatively, about 5 minutes, alternatively, about 10 minutes,alternatively, about 15 minutes, alternatively, about 20 minutes,alternatively, about 30 minutes, alternatively, about 40 minutes,alternatively, about 50 minutes, alternatively, about 60 minutes, ormore.

In an embodiment, the LCCC may be introduced into the wellbore, theformation, or an LCZ as a single pill fluid. That is, in such anembodiment, all components of the LCCC may be mixed and introduced intothe wellbore as a single composition. As will be understood by those ofskill in the art with the aid of this disclosure, introduction as asingle pill may be an appropriate mode of introduction where the settingof a gel can be delayed, retarded, or otherwise controlled such that thegel will not set until reaching a desired locale.

In an alternative embodiment, the LCCC may be introduced into thewellbore, the formation, or the LCZ in multiple components. As will beunderstood by those of ordinary skill in the art, it may be desirable oradvantageous to introduce components of the LCCC separately, forexample, in situations where the gellable composition of the LCCC willset within a relatively short time-frame (e.g., those gels which may setor begin to set within an amount of time less than is necessary tointroduce the LCCC into the desired LCZ). Introducing two or more of thecomponents of the LCCC separately allows the LCCC to be positionedwithin and LCZ prior to gelation. The separate introduction of at leasttwo of the LCCC components may be achieved by various means, describedin greater detail herein below.

In an exemplary embodiment, the benefits of separate introduction of thereactive components of the LCCC components may also be achieved byencapsulation of at least one of those components. For example, inaccordance with the compositions and methods of the instant disclosure,a single stream comprising a first component (e.g., a cross-linkingagent or initiator) may be encapsulated when introduced into thewellbore. The encapsulated component may be released so as to contactthe other components of the LCCC in a downhole portion of the wellborenear, proximate to, or within the LCZ. When the components of the LCCCare allowed to contact, the LCCC may gel or begin to gel. Thus, bycontacting the LCCC components within the LCZ, the gel will form withinthe LCZ.

In another exemplary embodiment, the separate introduction of at leasttwo of the LCCC components may be achieved by introduction via two ormore independent fluid streams. That is, a first component may beintroduced into the wellbore, formation, or LCZ via a first flowpath anda second component may be introduced via a second flowpath which isseparate from the first flowpath. The introduction of fluids into awellbore via two or more flowpaths is known to those of skill in theart, for example, via flow inside a tubular and an annular spaceddefined by the tubular and the wellbore. Introduction into the wellborevia two or more flowpaths may provide several advantages to theoperator. For example, the first component of the LCCC may be includedwithin a drilling fluid which is circulated through the wellbore duringdrilling operations. If an LCZ is encountered during drillingoperations, a second component may be introduced into the wellbore via aflowpath separate from the flowpath by which the drilling fluid iscirculated. Utilizing a gellable composition which sets instantaneouslyor substantially instantaneously causes gellation to occur where, orsubstantially near where, the first component and the second componentcome into contact. Thus, utilizing multiple flowpaths may allow theoperator to plug or seal an LCZ without entirely ceasing drillingoperations. By causing the LCCC within the drilling to set, the operatordoes not need to remove the drilling equipment and drilling fluid fromthe wellbore. The operator may thereby resume drilling operations morequickly, the LCZ having been plugged or sealed.

In still another exemplary embodiment, the separate introduction of atleast two of the LCCC components may be achieved by introducing thecomponents within a single flowpath, but being separated by a spacer.Such a spacer may comprise a highly viscous fluid which substantially orentirely prevents the intermingling of the LCCC components while beingpumped into a wellbore. Such spacers and methods of using the same aregenerally known to those of ordinary skill in the art. Once introducedto the subterranean formation the LCCC may enter the LCZ and set to forma mass that substantially inhibits or eliminates lost circulation.

In an embodiment, the LCCC introduced into the wellbore may be suitablefor preventing the loss of fluids to LCZs or other undesirable flowpathshaving particularly large voids, pores, spaces, fractures, or vugularzones. Such voids, pores, spaces, fractures, or vugular zones maycomprise cavities having diameters equal to or greater than about 200microns, alternatively equal to or greater than about 150 microns,alternatively equal to or greater than about 100 microns. Further, suchvoids, pores, spaces, fractures, or vugular zones may comprise slotswidths ranging from 500 to 5000 microns, alternatively, from 750 to 4500microns, alternatively, from 1000 to 4000 microns, alternatively, from1500 to 3500 microns. Such an LCCC will be referred to herein as aLarge-Pore LCCC (XC) and the voids, pores, spaces, fractures or vugularzones are hereinafter collectively referred to as cavities.

In an embodiment, the XC may comprise a base fluid a thixotropicviscosifier comprising a synthetic hectorite clay (e.g., THERMA-VIS) anda gellable composition comprising poly(acrylamide-co-acrylate) (HZ-10)and a polyethyleneimine crosslinker (HZ-20). The XC may also comprise ablend of particulate materials comprising resilient materials such asgraphite (e.g., Duo-Squeeze H). THERMA-VIS, HZ-10, HZ-20, andDUO-SQUEEZE H are commercially available from Halliburton EnergyServices, Inc.

In an embodiment, the method of combating lost circulation may compriseintroducing the XC into an LCZ or undesirable flowpath having cavitiesof the size described previously herein. The XC may be an effectivemeans of combating lost circulation in an LCZ or flowpath having poresof that size. The XC may be introduced as a single fluid or pill asdescribed previously herein. Not seeking to be bound by any particulartheory, the components of the XC may work synergistically to combat lostcirculation. For example, as the XC is introduced into the LCZ orflowpath, the bridging material may “bridge” some portion of thecavities equal to or greater than approximately the diameter or width ofthe LCZ or flowpath, thereby causing a reduction in the shear stressapplied to the XC. As the shear stress applied to the XC lessens, thethixotropic behavior of the thixotropic viscosifier causes the XC toviscosify. The increased viscosity of the XC due to the action of thethixotropic primary voscosifier will cause the XC to remain in placewhile the gellable composition begins to set. Thus an XC of the typedescribed herein may rapidly develop set gels at low or zero shear. Theset XC may form a ringing gel. As used herein, the term “ringing gel”means a substantially non-flowing gel with dimensional stability (e.g.,when a glass bottle containing the gel is tapped on a surface, thecontainer and the gel vibrate as a single entity). Thus, the synergisticaction of the components of the XC and methods of using the same may beparticularly advantageous for preventing the loss of fluid to an LCZ orflowpath having voids, pores, spaces, fractures, or vugular zones of thesize previously described herein. Further, the XC may be characterizedby thermal stability. As used herein, the term “thermal stability” meansthere is no loss in viscosity nor is syneresis experienced by excludingwater when kept at an elevated temperature (e.g., above about 300° F.).

In an embodiment, the LCCC introduced into the wellbore may be designedfor use in a situation where a solids-free or water-thin LCCC isdesired, referred to herein as a Solids-Free LCCC (YC). In anembodiment, the YC comprises a base fluid and a gellable composition.The base fluid may comprise at least 75% by weigh of the YC,alternatively, at least 90%, alternatively, at least 95%. The gellablecomposition may comprise less than 25% by weight of the YC,alternatively, less than 10%, alternatively, less than 5%.

In an embodiment, the base fluid of the YC comprises an aqueous fluid ora substantially aqueous fluid and, optionally, a salt. When present, thesalt may be present in an amount ranging from about 1% to a saturatedsolution.

In an embodiment, the gellable composition comprises a LVGS as describedherein above. In an embodiment, the LVGS suitably employed in the YCcomprises a soluble polymerizable monomeric acrylate of an alkalineearth metal, a multifunctional cross-linking unsaturated monomer, aredox polymerization initiator, a polymerization rate moderator, and,optionally, a redox polymerization rate retarder.

In another embodiment, the initiator of the LVGS comprises awater-soluble azo-initiator (e.g., PERM C™ and PERM D™, bothcommercially available from Halliburton Energy Services, Inc. In anembodiment, the YC may be advantageously employed where it is desirableto have a gel time within the foregoing ranges.

In an embodiment, the initiator of the LVGS comprises a combination ofalkoanolamine (e.g., triethanoloamine) and sodium or ammoniumpersulfate. When activated, the gellable composition may form a gel inan amount of time ranging form 0 to about 60 minutes at roomtemperature, alternatively, from about 30 seconds to 30 minutes,alternatively, from about 1 minute to about 20 minutes. The gel time maybe adjustable via the ratio of alkanolamine to persulfate or byemploying a retarder (e.g. potassium ferricyanide). The use of retardersto adjust gel time is generally known to those of skill in the art.

In an embodiment, the YC may be characterized as tolerant of highsalinity conditions. As used herein, the term “tolerant” means the geltime is not significantly altered and or no component of a compositionis precipitated upon exposure. In an embodiment, the YC may also beadvantageously employed where a brine or heavy brine is desired as thebase material (e.g., where the YC will be used, for instance, within orproximate to a salt dome).

In an embodiment, the YC may be characterized as water-thin.Alternatively, the YC may be characterized as having a viscosity of lessthan about 5 cP, alternatively less than about 10 cP, alternatively,less than about 15 cP. Furthermore, in an embodiment the YC may besubstantially free of solids. As used herein, substantially free solidsmeans that YC comprises less than about 10% by weight of any solid orparticulate component, alternatively less than about 9%, 8%, 7%, 6%, 5%,4%, 3%, 2% or 1% of a solid or particulate component. In an embodiment,the YC may be advantageously employed to combat lost circulation in anLCZ or a flowpath having pores ranging in size from greater than about200 microns, alternatively, greater than or equal to about 150 microns,alternatively, greater than or equal to about 100 microns. Not seekingto be bound by theory, relatively low viscosity of the YC comprisesallows the YC to be placed within a relatively small pore (e.g., a porewhere it would be difficult or impossible to place a higher viscosityfluid therein). In an embodiment, the YC may be advantageously employedto combat lost circulation in an LCZ or flowpath having pores ranging insize voids, pores, spaces, fractures, or vugular zones may compriseslots widths ranging from 500 to 5000 microns, alternatively, from 750to 4500 microns, alternatively, from 1000 to 4000 microns,alternatively, from 1500 to 3500 microns. In an embodiment, the YCcomponents are allowed to come into contact within an LCZ by introducingthe components into the wellbore as separate streams. The YC componentsmay be allowed to form a crosslinked gel. In an embodiment, thecrosslinked gel may form in about less than 5 min. In an embodiment, atleast one of the streams containing one reactive components contains athixotropic viscosifier. In an embodiment, at least one of the streamscontaining one reactive component contains a thixotropic viscosifier anda bridging particle.

The gel formed upon setting of the YC may be a highly rubbery gel havinggood dimensional stability. As will be understood by one of ordinaryskill in the art, use of a LVGS as described herein may provide for afinal product that can be characterized as having a reduced toxicitywhen compared to materials formed from cross-linking, non-ionic, watersoluble monomers with olefinic unsaturation.

In an embodiment, the method of combating lost circulation may compriseintroducing an YC into an LCZ having particularly small pores. Asdiscussed above, the YC will be free or substantially free of solid orparticulate materials. Furthermore, the YC may be substantiallywater-thin. Thus, not seeking to be bound by any particular theory, whenintroduced into an LCZ having particularly small pores, the YC may flowinto such small pores. As discussed above, the YC may set substantiallyinstantaneously or in a relatively short amount of time. Thus, the YCmay set quickly upon being introduced into the LCZ. Further, the YC maybe introduced into the wellbore, formation, or LCZ as two or morecomponents via the means discussed above. In such an embodiment, the YCmay set instantaneously or substantially instantaneously upon thecomponents thereof coming into contact.

In an embodiment, the method of combating lost circulation may compriseintroducing an YC into an LCZ within a salt dome (i.e., an area of wherethe formation comprises a relatively high proportion of saline crystals,often to the extent that the integrity of the formation would be alteredby the removal of the saline crystals). As will be understood by one ofskill in the art, when working in, near, or around salt domes, it may benecessary to utilize saturated or nearly saturated aqueous wellboreservicing fluid because an unsaturated aqueous wellbore servicing fluidcould cause the dissolution of the salt from the formation, therebyresulting in the loss of the structural integrity of the formation.Thus, it may be advantageous to utilize a saturated aqueous wellboreservicing fluid utilized in, near, around a salt dome. Alternately, saltsolutions may be useful in preventing clay swelling or shale swellingwhile drilling through zones comprising such materials. Alternately,salt solutions may be employed to increase density of LCCC. When asaturated aqueous solution is used as the base fluid of an LCCC, thegellable composition utilized therewith must tolerate salinity. In anembodiment, the gellable composition of the YC will tolerate highsalinity conditions and, thus, may be advantageously utilized with asaturated aqueous wellbore servicing fluid. As such, the YC may beutilized to combat lost circulation in, near, or around salt domes.

EXAMPLES Example 1

A YC was prepared by contacting a magnesium acrylate present in anamount of about 10% by weight of the final solution, triethanolamine,and a redox polymerization initiator and allowed to gel at roomtemperature. The amount of each component and the gel time for eachsample are presented in Table 1.

TABLE 1 Sample Acrylate Triethanolamine Redox Gel time at room No. (cc)¹Water (cc) (85%) in cc initiator (g)² temperature 1 10 30 0.67 0.8Immediate 2 10 30 0.34 0.8 2 min. 3 10 30 None 0.8 No gel in 18 hrs. 42.5 20 0.2 0.2 5 min. - weak gel 5 5 15 0.37 0.2 40 sec. 6 5 15 0.37 0.6No gel at room (encapsulated)³ temperature in 1 hr. Gelled at 165° F. in2 min. 7 5 15 0.37 0.4 Immediate (2.7% KCI) 8 5 15 0.18 0.4 2 min. (2.7%KCI) 9 3.5 10.5 0.25 0.3 (NaBO₃) 46 hrs. 10 3.5 10.5 0.25 0.3 (Na₂CO₃)18 hrs. 11 2.5 7.5 None 0.2 Less than 15 min. at 65° F. 12 2.5 7.5Polyethyleneimine⁴ 0.2 2 min. (0.2 cc) 13 2.5 7.5 0.2 cc + 0.4 cc of 1%0.2 4 min. K₃[Fe(CN)₆] ¹The Acylate (40% aqueous solution) utilized herewas AC-400 from De Neef Corporation. ²The redox polymerization initiatorutilized here was sodium persulfate (Na2S2O8) unless otherwise stated.³Encapsulated ammonium persulfate [(NH4)2S2O8] is available fromHalliburton Energy Services, Inc. as OPTIFLO II. ⁴Available fromHalliburton Energy Services, Inc. as HZ20 ™.

Referring to Sample Nos. 7 and 8, the results demonstrate that anaqueous salt solution can be employed in the YC without significantlyeffecting the gel time. The results also indicate that selection ofsuitable redox polymerization initiator such as sodium perborate orsodium percarbonate, extended gel times can be realized. Encapsulationof the oxidizer also provides prolonged fluid duration (e.g., as inSample No. 6). The results also demonstrate that polymeric amines can beused in place of triethanolamine. The resultant gels are thermallystable retaining their shape and as-gelled dimensions for at least 2weeks at 165° F. Additionally, Sample 13 demonstrates that aferricyanide salt may be employed in the composition to extend gel time.

Example 2

A YC was formed using a water-soluble azo initiator to form a singlefluid based pill. Samples 14 to 18 have the indicated compositions.

TABLE 2 Sample No. Acrylate (cc)¹ Water (cc) Azo Initiator (mg) Gel timeat F. 14 5 15 20 (Perm C) 15 min @ 150 F. 15 5 15 (4% KCl) 20 (Perm C)15 min @ 150 F. 16 5 15 (12% KCl + 5% MgCl₂•6H₂O) 20 (Perm C) 15 min @150 F. 17 5 15% (1% sodium acetate) 20 (Perm C) 15 min @ 150 F. 18 5 15%(1% sodium citrate) 20 (Perm C) 15 min @ 150 F. ¹AC-400 (40% solution inwater) from De Neef Corporation ²Small amounts of free water form in 24hrs in all cases.

PERM C is a low temperature initiator with a high temperature limit ofabout 140° F. to 150° F. Other high temperature water-solubleinitiators, for example azo initiators such as PERM D are also availablefor higher temperature applications. The results in Table 2 demonstratethat salt solutions containing monovalent or divalent cations may beemployed in the LCCCs of this disclosure without interfering with thegelation reaction. Similar results were observed with the methacrylatebased systems and these results are presented in Table 3.

TABLE 3 Sample Methacrylate Water Triethanolamine Redox Inititor Perm CAzo Gel time at room No (cc)¹ (cc) 85% in cc (sodium persulfate)(g)initiator (mg) temperature 1 10 10 .039 0.23 1 min 2 10 10 — — No geleven at 150 F. 3 10 10 — — 20 Gelled at 150 F. in 15 min ¹Superflex ™from De Neef Corporation

It was noted however that the methacrylate-based systems produced gelsthat were elastic and mechanically stronger than those produced by theacrylate-based systems.

Example 3

A 10 ppg XC was prepared using a base formulation comprising 3.0 wt. %VIS I, 2 wt. % VIS II. 2 X-LINK I, and 40 ppb DUO SQUEEZE H. DUO-SQUEEZEH is a high fluid loss circulation treatment, X-LINK I is a syntheticpolymer, VIS I is a synthetic macromolecule; and VIS II is a syntheticcopolymer; all of which are commercially available from HalliburtonEnergy Services, Inc. The initial rheological profile of the XC wasdetermined. At room temperature, the rheological properties of the fluidwere measured using a FANN 35 viscometer at 3, 6, 10, 100, 200, 300, and600 RPM. The results are shown in Table 4 and FIG. 2.

TABLE 4 FANN 35 150 F. 600 200 300 120 200 90 100 57  6 26  3 24 10 s 28

The results demonstrate the thixotropic viscosifier (Therma-vis) allowsfor the unique ability of the fluid to develop rapid gels at low or zeroshear. This behavior helps hold the fluid in place while the secondaryviscosifier (HZ-10) and cross-linker (HZ-20) thermally-activate to forma ringing gel and efficiently close off the loss zone. DUO-SQUEEZE H isadded as a sized mix of particles to aid in closing off of the zoneuntil the gelatin sets. The other unique aspect of the system is theability to tune the viscosity of the initial fluid; the gel set time,the gel strength and the activation temperature. It can be observed thatthe fluid exhibits very thixotropic behavior, but at low shear rates arapid gel increase is seen. Upon thermal application, a rigid gel formsand can be seen via dynamic studies with the Anton PAAR control stressrheometer.

While embodiments of the disclosure have been shown and described,modifications thereof can be made by one skilled in the art withoutdeparting from the spirit and teachings of the disclosure. Theembodiments described herein are exemplary only, and are not intended tobe limiting. Many variations and modifications of the disclosuredisclosed herein are possible and are within the scope of thedisclosure. Whenever a numerical range with a lower limit and an upperlimit is disclosed, any number and any included range falling within therange is specifically disclosed. In particular, every range of values(of the form, “about a to about b,” or, equivalently, “fromapproximately a to b,” or, equivalently, “from approximately a-b”)disclosed herein is to be understood to set forth every number and rangeencompassed within the broader range of values. Use of the term“optionally” with respect to any element of a claim is intended to meanthat the subject element is required, or alternatively, is not required.Both alternatives are intended to be within the scope of the claim. Useof broader terms such as comprises, includes, having, etc. should beunderstood to provide support for narrower terms such as consisting of,consisting essentially of, comprised substantially of, etc. Also, theterms in the claims have their plain, ordinary meaning unless otherwiseexplicitly and clearly defined by the patentee.

Accordingly, the scope of protection is not limited by the descriptionset out above but is only limited by the claims which follow, that scopeincluding all equivalents of the subject matter of the claims. Each andevery claim is incorporated into the specification as an embodiment ofthe present disclosure. Thus, the claims are a further description andare an addition to the embodiments of the present disclosure. Thediscussion of a reference herein is not an admission that it is priorart to the present disclosure, especially any reference that may have apublication date after the priority date of this application. Thedisclosures of all patents, patent applications, and publications citedherein are hereby incorporated by reference, to the extent that theyprovide exemplary, procedural, or other details supplementary to thoseset forth herein.

What is claimed is:
 1. A method of servicing a wellbore in asubterranean formation comprising: circulating a drilling fluid in thewellbore; losing a portion of the drilling fluid to a lost circulationzone in the subterranean formation, wherein the lost circulation zonecomprises cavities and the cavities comprise slots having widths rangingfrom 500 to 5000 microns; preparing a composition comprising a basefluid, a thixotropic viscosifier, a gellable composition and a bridgingmaterial; applying a shear force to the composition such that thecomposition viscosity decreases; introducing the composition into thelost circulation zone wherein the composition has a pump time of fromabout 30 min to about 48 hrs; decreasing the shear force applied to thecomposition; and allowing the composition to set in the lost circulationzone, wherein the thixotropic viscosifier comprises an associativepolymer thickener selected from the group consisting of acellulose-based polymer, an amorphous pyrogenic silica, an alkaliswellable latex, a polysaccharide, and combinations thereof, wherein thegellable composition gels by thermal activation and wherein thecomposition in the absence of the bridging material has a viscosity offrom about 5 centipoise to about 5000 centipoise.
 2. The method of claim1, wherein the composition is introduced to the lost circulation zone asa single pill.
 3. The method of claim 1, wherein the thixotropicviscosifier has a density of from about 0.5 g/cc to about 4.0 g/cc. 4.The method of claim 1, wherein the thixotropic viscosifier has aviscosity of from about 100 cp to about 2,000,000 cp.
 5. The method ofclaim 1, wherein the thixotropic viscosifier has a yield stress of fromabout 40 Pascals to about 40,000 Pascals.
 6. The method of claim 1,wherein the thixotropic viscosifier has a shear stress history responseof from about 1000 to about 10 million.
 7. The method of claim 1,wherein the gellable composition comprises polyacrylamide, acrylate andpolyethylene imine.
 8. The method of claim 1, wherein the bridgingmaterial comprises resilient graphite.
 9. The method of claim 1, furthercomprising removing the set composition by contacting the setcomposition with an acid.
 10. The method of claim 1, wherein thethixotropic viscosifier is present within the composition in an amountof from about 0.1% to about 20% by weight of the base fluid.
 11. Themethod of claim 1, wherein the gellable composition is present withinthe composition in an amount of from about 0.2% to about 30% by weightof the composition.
 12. The method of claim 1, wherein the compositionforms a ringing gel.
 13. The method of claim 1, wherein the gellablecomposition comprises a copolymer of acrylamide and t-butyl acrylate anda cross-linking agent comprising polyethylene imine.
 14. The method ofclaim 13, wherein the cross-linking agent is encapsulated.
 15. Themethod of claim 1, wherein the composition has a gel time of from about1 hour to about 96 hours.
 16. The method of claim 1, wherein theassociative polymer thickener is the cellulose-based polymer, and thecellulose-based polymer is a pseudoplastic.
 17. The method of claim 1,wherein the associative polymer thickener comprises a cellulose-basedpolymer, and the cellulose-based polymer is selected from the groupconsisting of cellulose ethers, methylcellulose, hypromellose,ethylhydroxycellulose, methylhydroxyethylcellulose, andhydroxypropylcellulose.
 18. The method of claim 1, wherein theassociative polymer thickener comprises a polysaccharide, and thepolysaccharide is selected from the group consisting of xanthan, diutan,gellan, gum tragacanth, and pestan.
 19. The method of claim 1, whereinthe associative polymer thickener comprises an amorphous pyrogenicsilica, and the amorphous pyrogenic silica has a diameter of less thanabout 100 nm.
 20. The method of claim 1, wherein introducing thecomposition into the lost circulation zone comprises introducingcomponents in a single flowpath separated by a spacer, wherein thecomponents are at least two of the base fluid, the thixotropicviscosifier, the gellable composition, and the bridging material. 21.The method of claim 1, wherein the base fluid comprises less than about50% of a nonaqueous component.
 22. The method of claim 1, wherein thebase fluid comprises an inorganic monovalent salt, a multivalent salt,or both.
 23. The method of claim 22, wherein the inorganic monovalentsalt, the multivalent salt or both are present in the base fluid in anamount of from greater than about 0% by weight to a saturated saltsolution.